1. Technical Field
The present invention relates to so-called scanning range sensors, which use a rotating or vibrating mirror or an equivalent to scan an object with a light beam within a predetermined angle range, and which receive light reflected by the object
2. Description of the Related Art
There are two different configurations, as shown respectively in FIGS. 5 and 6, known as a scanning range sensor that uses a rotating mirror for deflecting the axis of an optical beam over a full, 360-degree range of angles. Both configurations have a mirror whose optical axis coincides with the axis of a motor for rotating the mirror.
The configuration shown in FIG. 5 uses a motor 52 having rotary shafts 51a and 51b that constitute a common shaft and extend upward and downward, respectively. A scanning mirror 53 and a reflecting mirror 54 are attached to respective shafts 51a and 51b so as to have identical phase. At numeral 55 in FIG. 5 is a light transmitter, at numeral 56 is a photodetector, at numerals 57 and 58 are lenses, and at numeral 63 is a light exit/entrance window 63. There are two advantages to this configuration. One is that there is little diffraction of light from the scanning optics to the receiving optics, because the scanning optics and the receiving optics are completely separate from each other. Another advantage is that the photosensitivity of the sensor can be raised because there is no possibility of stray light being reflected by the inner surface of, or by dust particles on, the exit/entrance window 63 and then entering the photodetector 56.
The sensor section of the scanning range sensor configuration shown in FIG. 6 has a motor 52 with a rotary shaft 51c protruding upward and a light scanning/reflecting mirror 59 that is attached to the rotary shaft 51c. Light from the light transmitter 55 passes through a lens 60, is reflected downward by a half-silvered mirror 61, and enters the scanning/reflecting mirror 59. This reflected light is reflected by the scanning/reflecting mirror 59, passes through the half-silvered mirror 61, passes through a lens 62, and enters the photodetector 56. There are two advantages to this configuration. One is that there is no blind spot for an object even at a close distance from the sensor because one mirror 59 is used in common for both the scanning and the reflecting mirror. Another advantage is that when the sensor is installed in a device such as a robot, a high flexibility in installation can be obtained, for the reason just given.
However, there are some drawbacks to the configuration shown in FIG. 5.
First, since the motor 52 is positioned between the scanning mirror 53 and the reflecting mirror 54, the distance between the optical axes of the scanning optics and the receiving optics is large. Therefore, if a target object is positioned within close range, light reflected from the object does not enter the photodetector 56, resulting in the occurrence of a blind spot.
Secondly, the fact that the scanning and receiving optics are situated respectively above and below the motor renders the sensor as a unit elongate vertically. Meanwhile, the center of the two optical systems is coincident with the center of the sensor. Therefore, when installing the vertically elongate sensor in a device, its center must be the optical axis. This limits the degrees of freedom for installation in a device. In particular, if the sensor is to be installed in a low-profile device, a drawback is that the outward-extending portions are large.
In addition, there are a few drawbacks to the configuration shown in FIG. 6.
First, the half-silvered mirror 61 is employed to make the optical axes of the scanning optics and the receiving optics identical to each other. However, the amount of light is reduced by half after being separated by the half-silvered mirror 61. Therefore, the power of the laser in the light transmitter 55, and the amplifying capability of the photodetector 56 must be enhanced by four times in total compared with an implementation in which the scanning optics and the receiving optics are separated as shown in FIG. 5.
Secondly, the fact that the single scanning/reflecting mirror 59 is used means that the scanning beam may be reflected by the inner surface of, or by dust particles on, the light exit/entrance window 63, in which case the stray reflected light can enter the photodetector 56 after being transmitted by the scanning/reflecting mirror 59 and passing through the half-silvered mirror 61. If the photodetector 56 is of enhanced photosensitivity, the stray reflected light may generate noise that becomes added to image information. Consequently, the photosensitivity cannot be heightened without compensating for it.